The present invention relates to a hydrogen storage material and a process for producing the same. More particularly, it relates to a hydrogen storage material which is capable of electrochemically and reversibly storing and releasing hydrogen and shows reduced variations in storage characteristics and service life characteristics, which are important battery characteristics, and also exhibits excellent charge and discharge characteristics and a process for producing the same.
A hydrogen storage material used in a nickel metal hydride battery is used in the form of a hydrogen storage electrode which is generally prepared by packing a powdered hydrogen storage material, if desired in combination with a binder and a conductive agent, into a three-dimensional porous substrate or applying it to a two-dimensional substrate.
Hydrogen storage materials that are commonly and widely used in this type of electrodes are of hexagonal crystal form represented by MmNi5. These alloys usually have their alloying composition designed to secure characteristics required of electrodes, such as charge and discharge characteristics, storage characteristics, and service life characteristics. For example, the alloys have a high Co content, whereas alloy compositions with a reduced Co content have also been proposed (see JP-A-7-99055 and JP-A-9-213319). It has also been suggested to reduce the amount of Mn and Al dissolved out per specific surface area (see JP-A-11-96999).
With an alloy composition being fixed, however, stable battery characteristics cannot be maintained if the alloy structure varies. Hence it has been proposed to specify casting conditions or heat treating conditions in alloy production (see JP-A-11-152533 and JP-A-2000-234133). These manipulations alone nevertheless have difficulty in obtaining quite the same alloy structure, failing to stabilize the initial activity or discharge characteristics, particularly to completely suppress variations in storage characteristics and life characteristics.
In order to improve homogeneity of alloys thereby to improve characteristics, it has been proposed to atomize an alloy with a high-pressure inert gas or to feed a molten metal on the running surface of a rotating disk to finely scatter and solidify the molten metal to obtain spherical powder, or to drop a molten metal on a single roll rotating at a high speed to obtain a quenched thin ribbon. However, a large quantity of an inert gas which is expensive is required for obtaining a homogeneous hydrogen storage material powder. Moreover, the resulting spherical powder shows isotropic crystal orientation so that it suffers noticeable grain size reduction with repetition of hydrogen absorption and desorption.
On the other hand, in order to obtain a sufficient cooling rate in making a quenched thin ribbon, it is necessary to rotate the roll at a high speed. In this case, the solidified alloy obtained is as thin as 20 to 50 xcexcm and is inconvenient to handle in a subsequent heat treatment or grinding. Besides, because the solidification by cooling is unidirectional, the crystals are highly orientated only to have poor initial activity. The method has an additional problem of low productivity because the amount of the molten metal that can be dropped is limited considerably.
The orientation in an alloy could be controlled by reducing the cooling rate, which results in impairment of the alloy structure homogeneity.
The hydrogen storage materials heretofore proposed have thus encountered with difficulty in securing a service life and storage characteristics of the alloy in consistence with discharge characteristics.
Accordingly, an object of the present invention is to provide a hydrogen storage material which shows reduced variations in storage characteristics and life and yet exhibits excellent initial activity and charge and discharge characteristics and to provide a process of producing the same.
The above object of the present invention is accomplished by the following hydrogen storage material and the following process of producing the same.
The present invention provides a hydrogen storage material which is characterized by having, as a cast alloy, a flaky or similar shape with a thickness of 50 to 500 xcexcm and showing such crystallite orientation that the X-ray diffraction pattern has an intensity ratio of plane indices (002)/(200) of 2 to 10 as measured with the cooled surface of the flaky cast alloy being in parallel with a mount.
The present invention also provides a process for producing a hydrogen storage material as a preferred process for producing the hydrogen storage material of the present invention, which process is characterized by comprising dropping a molten metal in between at least two rotating rolls to make the molten metal fly three times or more between two of the rolls to rapidly solidify the molten metal by cooling.